In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipment (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition.
In 3GPP Release-11 LTE system, time-domain muting scheme together with interference cancellation receiver techniques are utilized for inter-cell interference coordination/cancellation to enable the cell range extension of picocells for better mobile data offloading from macrocell in the heterogeneous networks (HetNet), where there are deployments of both macrocells and picocells sharing the same frequency band. In addition, coordinated multi-point (CoMP) operation is also enabled to provide more system throughput gain with more tightly cooperation among base stations in HetNet. For further improvement of system throughput, wide deployment of small cells in the mobile networks is viewed as a promising technology and feature in 3GPP Release 12 LTE system. Small cells generally include picocells, hotspot, femtocells, and microcells in licensed band and WiFi AP in unlicensed band.
Unlike macrocell with a coverage radius ranging from one to several kilometers, small cells are low-power radio access nodes that operate in either licensed or unlicensed spectrum with a coverage radius ranging from tens to hundreds of meters. With emerging needs for more system throughput due to the popularity of smart phones, many mobile network operators are eagerly looking for methods to enhance the utilization efficiency of available radio spectrum by either spectrum efficiency improvement in licensed band or mobile data offloading in unlicensed band. As a technology providing promising gain in radio spectrum utilization efficiency, deployment of small cells receives broad attention from mobile network operators in recent years and 3GPP is planning to enable small cell deployment in the next release of LTE system.
In 3GPP Release-12 LTE system, the techniques to enable the deployment of small cells in licensed band will be the focus in RAN working groups. Due to possible acquisition of 3.5 GHz frequency bands, it enables the possibility of non-cochannel deployments for small cells to relief interference issues between macrocells and small cells. As one of considered scenarios, the signaling overhead of mobility management and the time radio access interruption due to handover can be improved by assigning the frequency band for the deployment of macrocells as a mobility layer and the other frequency band for the deployment of small cells as a capacity layer. In addition to non-cochannel deployment, further enhancements on the inter-cell interference coordination/cancellation techniques are also considered and under evaluation for cochannel deployment.
With large number of small cell deployments, new techniques are needed to resolve possible issues in both protocol and physical layers, such as mobility management, inter-cell interference handling, on/off small cell operation, etc. In addition, how to enable smooth migration of legacy UEs with limited impact is also one of important issues. The technique for efficient small cell discovery is one of the techniques remain under evaluation and development.
To support small cell on/off operation for the mitigation of inter-cell interference due to cell-specific reference signals (CRS) and load shifting among small cells, discovery of multiple small cells within limited time is needed and discovery reference signal (DRS) was proposed in 3GPP to enable it. One of candidate solutions for discovery signal design is to reuse existing reference signal designs. However, they suffer from high inter-cell interference level or large reference signal overhead. In addition, timing/frequency synchronization offset between small cells also affects the performance of cell detection and measurement using some existing reference signal designs. Therefore, enhancements are needed to resolve aforementioned problems when existing reference signal design is reused for small cell discovery.